Logic Board LEDs: Power Mac G4 (all models except PCI
Graphics)
There is one red LED on the Power Mac G4 (AGP Graphics/Gigabit
Ethernet/Digital Audio/QuickSilver/QuickSilver 2002) logic
boards. It indicates that there is power to the board and does not
imply a fault condition. Hardware such as DIMMs and PCI cards
should not be installed or removed when the LED is on.
https://archive.org/stream/powermac_g4/powermac_g4_djvu.txt